Field of the Invention
The invention relates to an apparatus and a method for determining a stray magnetic field in the vicinity of a sensor.
Description of the Related Art
The contactless measurement of a rotation angle with the aid of the Hall effect is known in the art. For example, an apparatus is known, from the publication by Reymond, S. et al., “True 2-D CMOS Integrated Hall Sensor”, IEEE SENSORS 2007 Conference, pages 860-863, for the contactless measurement of a rotation angle, having a semiconductor substrate in which 64 so-called vertical Hall sensors as magnetic field sensors are integrated. The magnetic field sensors of this publication are arranged at equally-spaced intervals along a circular path disposed in the chip plane of the semiconductor substrate. The vertical planes, in which the magnetic field sensors are positioned, are each arranged radially to a notional center axis extending through the center of circle of the circular path and the vertical planes are positioned orthogonally to the chip plane. The magnetic field sensors are connected to a scanning device such that the measurement signals from the individual magnetic field sensors are connected consecutively to a differential output connector to produce a rotation scanning signal. The values from the magnetic field sensors are thus read out in cyclically rotating fashion.
Metz et al. also describe the contactless measurement of a rotation angle in a publication “Contactless Angle Measurement using Four Hall Devices on Single Chip”, Transducers, 1997 International Conference on Solid-State Sensors and Actuators, Chicago, 16-19 Jun. 1997. This publication shows four lateral Hall sensors arranged at equally spaced intervals on a circular path disposed in the chip plane of the semiconductor substrate. A permanent magnet having two poles is attached at the end of a rotating axle and generates a magnetic field in the Hall sensors. The same apparatus is described in the European Patent Publication No. EP-B-0 916 074.
The known apparatuses have in common that a permanent magnet is mounted on a rotatable element and generates a magnetic field that is captured by the Hall sensor. One issue in such apparatus is the presence of any stray magnetic fields or interference fields in the surroundings of the Hall sensor. A signal processing processor needs to compensate the measured values received from the Hall sensors in order to compensate for these stray magnetic fields. This compensation is possible in a relatively simple fashion for a substantially homogeneous background field. The compensation for a magnetic field that is generated by current flowing in a nearby conductor is more difficult, since the calculation of the compensation also has to consider the field gradient of the magnetic field generated by the current in the nearby conductor.
Modern cars have numerous ones of such current-carrying conductors causing such stray magnetic fields. A complete shielding of these stray magnetic fields in the vicinity of a rotation-angle measurement device to avoid interference from the stray magnetic fields is impossible.
The effects of the stray magnetic fields can be substantially eliminated by using a difference determination of the measurements of the magnetic field (signal field plus the contribution of the stray magnetic field) from several ones of a plurality of Hall sensors. This determination is also known in the state of the art. One arrangement for this suppression is the use of a diametric magnet having four lateral Hall sensors arranged on a circle. It is possible to compute the X component and the Y component of a magnetic rotation vector of the stray magnetic field from the difference determination of the measurements from two of these Hall sensors.
A further known solution utilizes a four-pole magnet having four vertical Hall sensors which, combined as an X/Y pixel cell, are disposed on a line at an equal distance from each other. The magnetic rotation vector of the stray magnetic field can also be computed using this solution.
In these prior art solutions, the suppression of the stray magnetic field caused by a current-carrying conductor with a current of 400 A at a predetermined distance (e.g. 2.5 cm) is around 51 dB, when the field strength of the permanent magnet is around 30 mT. This interference due to the stray magnetic field would lead however to a maximum angle error of around 0.5° in the measurement of a rotation angle using the four-pole magnet known in the prior art.
A further solution is known from the European Patent Application No EP 1 775 501 A1 which teaches a position sensor for use in an angular sensor for detecting the position of a selection lever in automatic gears. The magnetic field is measured by two sensor elements which enable a correction of any errors due to the presence of stray magnetic fields.
The German Patent Application teaches a magnetic field generating unit which generates a rotating magnetic field with a first partial magnetic field in a first position and a second partial magnetic in a second position. A detection unit can detect a first and second angle at the corresponding first and second positions. Using the values of the first and second angles, an angle value can be calculated which is representative of the direction of the rotating magnetic field.